In general, an eddy-current retarding device (hereinafter, also simply referred to as a “retarding device”) employing a permanent magnet (hereinafter, also simply referred to as a “magnet”) includes a brake member fixed to a rotating shaft such as a propeller shaft, and at the time of braking, causes eddy current to be generated on the surface of the brake member opposite to the magnet, due to an effect of a magnetic field from the magnet. With this eddy current generated, braking force occurs in a direction opposite to the rotational direction of the brake member rotating integrally with the rotating shaft, thereby reducing the speed of the rotation shaft.
Retarding devices are roughly divided into a drum type and a disk type according to the shapes of a brake member that causes braking force by generating eddy current and the shapes of a magnet holding member that holds a magnet and is paired with the brake member, and there are various structures for switching from braking to non-braking and vice versa.
In recent years, in order to respond to requests for miniaturized devices, there have been proposed retarding devices that rotatably support, on a rotating shaft, a magnet holding member that holds the magnet, and brings the magnet holding member to a stop with a friction brake at the time of braking (see, for example, Patent Documents 1 to 5). Furthermore, there is proposed a retarding device in which, by replacing the brake member with the magnet holding member, the magnet holding member is fixed on the rotating shaft, and the brake member is rotatably supported on the rotating shaft, thereby stopping this brake member with the friction brake at the time of braking (see, for example, Patent Document 5). These retarding devices are called retarding devices with a synchronous rotation type, because the magnet holding member and the brake member synchronously rotate at the time of non-braking periods as described below.
FIG. 1 is a longitudinal sectional view showing a configuration example of a conventional retarding device with a synchronous rotation type. The retarding device shown in FIG. 1 is a disk-type retarding device, and includes a brake disk 101 serving as a brake member, and a magnet holding disk 104 that serves as a magnet holding member and holds a permanent magnet 105 so as to face the main surface of the brake disk 101.
In FIG. 1, the brake disk 101 is configured so as to rotate integrally with a rotating shaft 111 such as a propeller shaft. More specifically, a connecting shaft 112 is fixed with a bolt or other items so as to be coaxial with the rotating shaft 111, and a sleeve 113 with a flange is inserted into the connecting shaft 112 while being engaged using a spline, and is fixed with a nut 114. The brake disk 101 is fixed, for example, with a bolt to the flange of the sleeve 113 attached integrally with the rotating shaft 111, which makes it possible to rotate integrally with the rotating shaft 111.
The brake disk 101 is provided with radiating fins 102 on, for example, the outer circumference of the brake disk 101. These radiating fins 102 are formed integrally with the brake disk 101, and have a function of cooling the brake disk 101 itself. The brake disk 101 is formed with an electrically conductive material, which includes a ferromagnetic material such iron, a soft magnetic material such as ferritic stainless steel, and a non-magnetic material such as aluminum alloy and copper alloy.
In FIG. 1, the magnet holding disk 104 is configured so as to be able to rotate with respect to the rotating shaft 111. The magnet holding disk 104 may be integrally formed with a ring-shaped member 103 that is coaxial with the connecting shaft 112, or may be formed separately and be fixed to the ring-shaped member 103, for example, with a bolt. The ring-shaped member 103 is supported through bearings 115a and 115b by the sleeve 113 attached integrally with the rotating shaft 111. With this configuration, it is possible for the magnet holding disk 104 to rotate relatively to the rotating shaft 111. The bearings 115a and 115b are filled with lubricating grease. This lubricating grease is prevented from leaking by ring-shaped seal members 116a and 116b attached on both ends of the ring-shaped member 103 in the front and rear direction.
On a surface of the magnet holding disk 104 opposite to the main surface of the brake disk 101, plural permanent magnets 105 are fixed in the circumferential direction. Each of the permanent magnets 105 is oriented in a manner such that a direction of magnetic poles (north pole or south pole) is in an axial direction of the magnet holding disk 104, and the permanent magnets 105 are arranged in a manner such that magnetic poles of magnets adjacent in the circumferential direction are alternately different from each other.
In FIG. 1, to the magnet holding disk 104, a magnet cover 120 made out of a thin sheet is attached so as to cover the entire permanent magnets 105. This magnet cover 120 protects the permanent magnets 105 from iron powder or dust particles, and at the same time, provides a function of shielding radiant heat coming from the brake disk 101 to the permanent magnets 105, thereby suppressing a reduction in magnetic force of each of the permanent magnets 105 due to thermal effects. The magnet cover 120 is made out of a non-magnetic material so that the magnetic field does not suffer from any effect from the permanent magnets 105.
The retarding device shown in FIG. 1 includes a disk brake serving as a friction brake that stops the magnet holding disk 104 at the time of braking. This disk brake is disposed at the rear of the magnet holding disk 104, and is configured to include a brake disk 106 formed integrally with the ring-shaped member 103, a brake caliper 107 having brake pads 108a and 108b located at both sides of this brake disk 106, and an electrically driven direct-acting actuator 109 that drives this brake caliper 107. The brake disk 106 is attached to the ring-shaped member 103, for example, with a bolt, and is attached integrally with the ring-shaped member 103.
The brake caliper 107 has a pair of the brake pads 108a and 108b at the front and the rear therein. Between the brake pads 108a and 108b, the brake disk 106 is disposed to face each other with a predetermined gap therebetween, and the brake caliper 107 is pressed and supported toward the bracket 117, for example, with a bolt having a spring. This bracket 117 is attached to a non-rotating portion such as a chassis and a crossmember of a vehicle. Furthermore, the bracket 117 surrounds the ring-shaped member 103 at a position more rearward than the brake disk 106, and is supported by the ring-shaped member 103 through a bearing 118 in a rotatable manner. This bearing 118 is filled with lubricating grease. Leakage of this lubricating grease is prevented by ring-shaped seal members 119a and 119b attached on both ends of the bracket 117 in the front and rear direction.
The actuator 109 is fixed to the brake caliper 107, for example, with a bolt. The actuator 109 is actuated with an electrically driven motor 110, and converts rotary motion by the electrically driven motor 110 to linear motion, thereby linearly moving the brake pad 108b on the rear side toward the brake disk 106. With this movement, the brake pad 108b on the rear side presses the brake disk 106. With an effect of the resulting counterforce, the brake pad 108a on the front side moves toward the brake disk 106, so that the brake disk 106 is strongly squeezed by the brake pads 108a and 108b on the front and the rear sides.
In the retarding device shown in FIG. 1, the disk brake (friction brake) is not activated during non-braking periods. At this time, in the case where the brake disk 101 is made out of a ferromagnetic material or a soft magnetic material, as the brake disk 101 rotates integrally with the rotating shaft 111, the magnet holding disk 104 integrated with the ring-shaped member 103 rotates synchronously with the brake disk 101 due to a magnetic attraction effect between the permanent magnet 105 and the brake disk 101. With this configuration, there occurs no difference in relative rotational speed between the brake disk 101 and the permanent magnet 105, and hence, braking force does not occur.
In the case where the brake disk 101 is made out of a non-magnetic material, the magnetic attraction force does not act between the magnet 105 and the brake disk 101. However, in association with the brake disk 101 rotationally moving in a magnetic field from the magnet 105, braking force acts on the brake disk 101 due to the effect of the magnetic field. Thus, the magnet 105 receives the resulting counterforce, and rotates in the direction same as the brake disk 101. More specifically, the magnet 105 rotates at a relative rotational speed slightly differing from that of the brake disk 101 rotating in the same direction so as to maintain a balance between the braking force generated as a result of the difference in relative rotational speed between the brake disk 101 and the magnet 105, and a loss occurring at a bearing portion due to rotation of the magnet 105 or drag force related to air resistance caused by rotation of the magnet holding disk 104. In other words, in the case where the brake disk 101 is made out of a non-magnetic material, the magnet 105 does not rotate in a fully synchronized manner with the brake disk 101 but substantially synchronously rotates with a slight difference in rotational speed, whereby non-braking state is maintained.
On the other hand, at the time of braking, the disk brake (friction brake) is caused to activate to make the brake disk 106 squeezed by the brake pads 108a and 108b. With this operation, the magnet holding disk 104 formed integrally with the ring-shaped member 103 stops rotating, and the magnet holding disk 104 is brought to a stop. If only the magnetic holding disk 104 is brought to a stop when the brake disk 101 is rotating, a difference in relative rotational speed takes place between the brake disk 101 and the permanent magnet 105. This causes eddy current to be generated on the main surface of the brake disk 101 due to an effect of a magnetic field from the permanent magnet 105, whereby it is possible to cause the braking force to act on the rotating shaft 111 through the brake disk 101. Note that, during braking periods, the same principle, involving the effect of the magnetic field, applies regardless of whether the brake disk 101 is made out of a ferromagnetic material or non-magnetic material, and braking efficiency differs due to a difference in electrical conductivity or magnetic permeability between materials, which makes it possible to appropriately select materials for the brake disk 101 at the time of designing magnetic circuits.
As described above, the retarding device shown in FIG. 1 has a configuration in which the brake disk 101 serving as the brake member is connected to the rotating shaft 111, and the magnet holding disk 104 serving as the magnet holding member is rotatably supported on the rotating shaft 111. However, it may be possible to employ a configuration in which the brake disk 101 and the magnet holding disk 104 are interchanged with each other. More specifically, it may be possible to employ a configuration in which the magnet holding disk 104 is fixed to the rotating shaft 111, and the brake disk 101 is rotatably supported on the rotating shaft 111.
In the case of this retarding device, during non-braking periods, as the magnet holding disk 104 rotates integrally with the rotating shaft 111, the brake disk 101 integrated with the ring-shaped member 103 rotates in synchronization with the magnet holding disk 104 due to the magnetic attraction effect (in the case where the brake disk 101 is made out of a magnetic material) with the permanent magnet 105 held by the magnet holding disk 104, or the effect of a magnetic field (in the case where the brake disk 101 is made out of a non-magnetic material). For this reason, there occurs no difference in relative rotational speed between the brake disk 101 and the permanent magnet 105 of the magnet holding disk 104, and hence, the braking force does not occur.
On the other hand, at the time of braking, the ring-shaped member 103 stops rotating due to operation of the disk brake, and the brake disk 101 is brought to a stop. If only the brake disk 101 is brought to a stop when the magnet holding disk 104 is rotating, a difference in relative rotational speed takes place between the brake disk 101 and the permanent magnet 105 of the magnet holding disk 104. This causes eddy current to be generated on the main surface of the brake disk 101. Consequently, braking force in a direction opposite to the rotational direction of the magnet holding disk 104 rotating takes place in accordance with the Fleming's left-hand rule based on the interaction between the eddy current generated on the main surface of the brake disk 101 and magnetic flux density from the permanent magnet 105, whereby it is possible to reduce the speed of rotation of the rotating shaft 111 through the magnet holding disk 104.
Furthermore, in the description of the retarding device with a synchronous rotation type above, a disk type has been described. However, the same description applies to the case of a drum type.